Recently, the necessity for lightweight automobiles has significantly increased to address environmental problems by reducing the emission of exhaust gases causing the greenhouse effect and improving the fuel efficiency of automobiles. Here, the use of high-strength steels is effective in reducing the weight of car bodies. However, if there is a lower limit to the thickness of automotive steel sheets to satisfy stiffness requirements of structural members, even if high-strength steel sheets are used, it may be difficult to reduce the weight of automobiles because the thickness of the high-strength steel sheets cannot be reduced below the lower thickness limit.
As a method of realizing weight reductions, aluminum alloy sheets having a specific gravity lower than that of steel sheets may be used. However, aluminum alloy sheets are expensive and have low workability compared to steel sheets, and it is difficult to weld aluminum alloy sheets to steel sheets. Therefore, the application of aluminum alloy sheets to automobiles is limited.
High-aluminum steels made by adding aluminum to iron in large amounts have a high degree of strength and a low degree of specific gravity and are thus theoretically effective in reducing the weight of automotive components. However, it is practically difficult to use high-aluminum steel sheets as automotive steel sheets that should have both high strength and high formability because of characteristics of high-aluminum steel sheets such as: (1) poor manufacturability, for example, cracking during a rolling process, (2) a low degree of ductility, and (3) the necessity of complicated heat treatment processes.
Particularly, it is theoretically possible to reduce the weight of steel sheets by increasing the content of aluminum (Al). In this case, however, the ductility, hot workability, and cold workability of such steel sheets are markedly decreased because of the precipitation of intermetallic compounds such as Fe3Al, having a DO3 structure, or FeAl, having a B2 structure. Furthermore, if manganese (Mn) and carbon (C), austenite stabilizing elements, are added to the steel sheets in large amounts so as to suppress the formation of intermetallic compounds, κ-carbide ((Fe,Mn)3AlC), a perovskite carbide having an L12 structure may precipitate in large amounts, and thus the ductility, hot workability, and cold workability of such steel sheets may be markedly decreased. Therefore, it is difficult to manufacture such high-aluminum steel sheets through general steel sheet manufacturing processes or to impart proper degrees of strength and ductility to such high-aluminum steel sheets.
In this regard, Japanese Patent Application Laid-open Publication No. 2005-120399 discloses a technique for improving the ductility and rollability of a high specific strength steel by adding aluminum, the high specific strength steel including, by wt %, C: 0.01% to 5%, Si<3%, Mn: 0.01% to 30%, P<0.02%, S<0.01%, Al: 10% to 32%, and N: 0.001% to 0.05%, wherein the high specific strength steel includes at least one optional element selected from Ti, Nb, Cr, Ni, Mo, Co, Cu, B, V, Ca, Mg, a rare earth metal (REM), and Y, and a balance of Fe. In addition, Japanese Patent Application Laid-open Publication No. 2005-120399 discloses a method of preventing grain boundary embrittlement caused by the precipitation of intermetallic compounds such as Fe3Al and FeAl in high-aluminum steel having an aluminum content greater than 10% by (1) optimizing hot rolling conditions to suppress the precipitation of intermetallic compounds such as Fe3Al and FeAl during hot rolling, cooling, and coiling processes, (2) suppressing the embrittlement of the high-aluminum steel by minimizing the contents of sulfur (S) and phosphorus (P) and inducing grain refinement using fine carbonitrides, and (3) guaranteeing manufacturability by adding chromium (Cr), cerium (Ce), and boron (B) if it is difficult to suppress the precipitation of intermetallic compounds. However, there is no way to confirm improvements in rollability by these techniques. In addition, according to the techniques, a low degree of yield strength may be obtained, and ductility may be only slightly increased. Thus, the application of the techniques to automotive members is limited.
In addition, for example, as a technique for improving the ductility and rollability of a high-aluminum steel sheet and improving manufacturability to manufacture the high-aluminum steel sheet through general thin steel sheet manufacturing processes while imparting satisfactory strength-ductility characteristics to the high-aluminum steel sheet, Japanese Patent Application Laid-open Publication No. 2006-176843 discloses a high specific strength steel including aluminum (Al) and a method for manufacturing the high specific strength steel, the high specific strength steel including, by wt %, C: 0.8% to 1.2%, Si<3%, Mn: 10% to 30%, P<0.02%, S<0.02%, Al: 8% to 12%, and N: 0.001% to 0.05%, wherein the high specific strength steel includes at least one optional element selected from Ti, Nb, Cr, Ni, Mo, Cu, B, V, Ca, Mg, Zr, and a REM, and a balance of Fe. The disclosed technique proposes a method of improving the ductility of steel having a high weight percentage of aluminum (Al) within the range of 8.0% to 12.0% by (1) adding carbon (C) in an amount of 0.8% to 1.2% and manganese (Mn) in an amount of 10% to 30% to form an austenite matrix (area fraction >90%), and (2) optimizing manufacturing conditions to suppress the precipitation of ferrite and κ-carbide ((Fe,Mn)3AlC) (ferrite: 5 area % or less, κ-carbide: 1 area % or less). However, since the steel proposed in the disclosed technique has a low degree of yield strength, there are limitations in applying the steel to automotive members requiring impact resistance.
For example, as a technique for improving the ductility and rollability of a high-aluminum steel sheet and improving manufacturability to manufacture the high-aluminum steel sheet through general thin steel sheet manufacturing processes while imparting a satisfactory strength-ductility level to the high-aluminum steel sheet, Japanese Patent Application Laid-open Publication No. 2006-118000 discloses a high specific strength steel including aluminum (Al) and a method for manufacturing the high specific strength steel, the high specific strength steel including, by wt %, C: 0.1% to 1.0%, Si<3%, Mn: 10% to 50%, P<0.01%, S<0.01%, Al: 5% to 15%, N: 0.001% to 0.05, wherein the high specific strength steel includes at least one optional element selected from Ti, Nb, Cr, Ni, Mo, Co, Cu, B, V, Ca, Mg, an REM, and Y, and a balance of Fe. The disclosed technique proposes a method of improving a strength-ductility balance by adjusting phase fractions of a metal microstructure and forming a composite microstructure of ferrite and austenite.
For example, as a technique for improving the ductility and rollability of a high-aluminum steel sheet for automobiles and improving manufacturability to manufacture the high-aluminum steel sheet through general thin steel sheet manufacturing processes while imparting a satisfactory strength-ductility level to the high-aluminum steel sheet, Japanese Patent No. 4235077 discloses a high specific strength steel including aluminum (Al) and a method for manufacturing the high specific strength steel, the high specific strength steel including, by wt %, C: 0.01% to 5.0%, Si<3%, Mn: 0.21% to 30%, P<0.1%, S<0.005, Al: 3.0% to 10%, N: 0.001% to 0.05%, wherein the high specific strength steel includes at least one optional element selected from Ti, Nb, Cr, Ni, Mo, Co, Cu, B, V, Ca, Mg, an REM, Y, Ta, Zr, Hf, W, and a balance of Fe. The disclosed technique is basically for improving toughness by suppressing grain boundary embrittlement. To this end, the disclosed technique proposes a method of manufacturing a high specific strength steel sheet (having a strength of 440 MPa or greater) by (1) markedly reducing the contents of sulfur (S) and phosphorus (P), (2) properly adjusting the content of carbon (C) to ensure manufacturability, and (3) limiting the contents of heavy elements.
For example, as a technique for reliably manufacturing a high specific strength steel sheet having a high aluminum content, Japanese Patent Application Laid-open Publication (Translation of PCT Application) No. 2006-509912 discloses a high specific strength steel including aluminum (Al) and a method for manufacturing the high specific strength steel, the high specific strength steel including, by wt %, C: 1% or less, Mn: 7.0% to 30.0%, Al: 1.0% to 10.0%, Si: from greater than 2.5% to 8%, Al+Si: from greater than 3.5% to 12%, B<0.01%, Ni<8%, Cu<3%, N<0.6%, Nb<0.3%, Ti<0.3%, V<0.3%, P<0.01%, and a balance of inevitable impurities and Fe. According to the disclosed technique, after general processes for manufacturing a steel strip and a steel sheet, a room-temperature forming process is performed to adjust the yield strength of a final steel product. The disclosed technique is for twinning-induced plasticity (TWIP) steels.